Xbox Evolved writes: "The AI Director - Valve's own behind-closed-doors game puppeteer - takes center stage and makes for a terrifying good time with friends who you'll need to help up, heal up, and not shoot. After all, friendly fire (in the upper difficulty levels, anyway) is lethal.
Playing with friends is where the game shines. Expectations of coming in and playing it as a single-player game should be put in check right from the start. L4D is fully playable by yourself but can be incredibly boring, especially played on normal, and short. The "story" (spanning four movies with five acts each) can be plowed through in 3-4 hours. "